Adds publish/subscribe pattern to allow plugins and projects to hook into events (#55)

Also adds CLI to ease development.

packages/medusa-core-utils/src/get-medusa-version.js

@@ -0,0 +1,14 @@
+import path from "path"
+
+export const getMedusaVersion = () => {
+  try {
+    return require(path.join(
+      process.cwd(),
+      `node_modules`,
+      `@medusajs/medusa`,
+      `package.json`
+    )).version
+  } catch (e) {
+    return ``
+  }
+}